In a conventional wireless communication network, base stations (called “macro base stations”, or “macros” for short), at which cell towers are normally located, are typically set up after a lot of planning, and have known positions. However, in indoor locales, signals from macro base stations are typically weaker or subject to multipath. To address such issues, wireless service can be provided in an indoor environment (e.g. within a building or inside a mine) by use of base stations that are designed to have low power (called “femto base station” or simply “femto”) relative to a macro base station (normally located outdoors).
Accordingly, a femto base station (also called an Access Point Base Station, femtocell, Home NodeB (HNB), Home Evolved NodeB (HeNB), or femto for short) is an indoor base station. Like a normal (or macro) base station, a femto connects cell phone voice and data to the cell phone network, but it serves a smaller area. Femtos are normally deployed by users themselves, e.g. in a home or in an office building. Use of a femto base station benefits a wireless service provider because it offloads cell tower traffic. Subscribers benefit from superior signal strength, due to the proximity of the unit, especially in environment where a cellular signal from normal base stations is weak or not available.
Femtos are normally connected using a wired connection to a wireless communication network e.g., over the public Internet through a backhaul line, such as a digital subscriber line (DSL) or cable modem connection. One issue with locating a femto in an indoor environment may be that whenever a femto is installed, the geographic location of that femto may not be immediately known to a network operator of a wireless service provider. Moreover, users may move femtos within their homes, or may carry the femtos with them e.g., when relocating or traveling. The geographic locations, where users have installed their femtos, need to be known to an operator in order to meet various regulatory mandates and business interests.
Conventional methods can determine geographic coordinates of a measuring station using observed time difference of arrival (OTDOA) values to compute position, when the femto can receive signals from macro base stations. Normally, each OTDOA value may be computed as a difference between two time measurements, namely a time of arrival (TOA) of a signal that is transmitted by a macro base station at a known position in a vicinity (“neighbor base station”) of the measuring station, and another TOA of another signal from another macro base station also having a known position (“reference base station”).
The above-described OTDOA values normally include two time components as follows (a) one time component arises from the difference in distance between two macro base stations (also called “Geometric Time Difference” or GTD) and (b) another time component arises from a synchronization offset between the macro base stations (also called “Relative Time Difference” or RTD). Hence, OTDOA can be expressed as: OTDOA=GTD+RTD. Note that location information is present only in the GTD, i.e. not in the RTD. Thus, GTD can be expressed as: GTD=OTDOA−RTD. In order to calculate the position of a station that made the measurements, the following need to be known: (a) OTDOA values obtained from TOA measurements performed at a femto, (b) the coordinates of the neighbor base station(s) and the reference base station(s), and (c) the synchronization offset between the base stations (RTDs).
RTDs may be computed in an asynchronous network by use of devices that are called Location Measurement Units (LMUs), which may be specifically deployed in the network to measure RTDs between pairs of base stations at known positions. Each LMU may determine OTDOA values in the above-described manner, and then use its known distance to the base stations to compute RTD as follows RTD=OTDOA−GTD wherein OTDOA is measured by the LMU, and the GTD is known because base station positions and LMU positions are known. In a synchronous network, transmissions from base stations may be synchronized to a common clock (e.g, Global Positioning System (GPS) time, or Global Navigation Satellite System (GNSS) time), and therefore, RTDs may be known. For example, when all transmission frames of a base station are synchronized to the same time, RTDs are zero.
To summarize the above description, coordinates of a measuring station can be determined by use of (1) OTDOA measurements made at that measuring station using transmissions from macro base stations, (2) RTDs between the macro base stations and (3) coordinates of the macro base stations.
However, signals from macro base stations may not be measurable by a newly-installed femto, using a conventional OTDOA method for several reasons. In order to calculate a location using the OTDOA method, signals from at least three macro base stations that have known locations must be received, and their RTDs must be made known to the newly-installed femto, by some means. Since femtos are typically deployed indoors, signal reception from three macro base stations can generally not be assumed, in particular not in environments where poor macro cell coverage is the cause for deployment of that newly-installed femto. In an indoor environment, the signal reception from GPS/GNSS satellites is also usually limited or of such poor quality as to make it difficult to determine the geographic coordinates of an indoor femto using a satellite-signal based position determining method.